Magnetic bubble memories are known to operate in a familiar field-access mode. Memories of this type are characterized by a pattern of magnetic elements, typically of magnetically soft permalloy, which responds to a magnetic field rotating in a plane of bubble movement to generate a pattern of localized magnetic field gradients. A magnetic bias field, antiparallel to the magnetization of a bubble and normal to the plane of bubble movement, is present to provide a range of stability over which a bubble memory is operable.
A highly attractive feature of a bubble memory is its nonvolatility. That is to say that a bubble pattern representative of information remains undisturbed in the absence of a drive field. Thus, power to a bubble memory may be interrupted without the loss of information
Yet information may be disturbed in a bubble memory during the transients which occur when the drive field stops and starts. For example, a bubble pattern may occupy a particular set of positions with respect to the permalloy pattern when the drive field stops and the field may later start in an orientation consistent with the occupation of different positions. Ambiguous movement of information results. Consequently, it has been found to be helpful if a power interruption occurs, for the drive circuitry to be adapted to ensure movement of information to closest safe positions and that the drive field next be activated in an orientation consistent with the occupation of those positions.
It has long been known that an in-plane (holding) field improves the stability of bubble patterns during stop-start operation. Such a field is oriented in a direction, in the plane of bubble movement, consistent with the occupation of safe positions by a bubble pattern. The result is improved operating margins and yield in the manufacture of bubble packages.
Because the presence of the holding field has been generally thought to be helpful only during stop-start operation, its presence during normal operation was considered of little if any positive value. Because that presence causes a variation in the drive field amplitude, its elimination during operation has been thought desirable. Thus, U.S. Pat. No. 3,744,042 issued July 3, 1973 discloses the canting of the magnetic bias field-generating means with respect to the plane of bubble movement in order to provide a holding field. But the patent also shows a circuit powered by the drive field power supply which provides a field equal and antiparallel to the holding field. During normal operation, the net result is that these two fields ideally cancel. Yet when drive power terminates, so does the antiparallel field leaving the holding field to ensure bubble stability.
The canting of the bias magnet with respect to the plane of bubble movement requires considerable attention for each device so arranged. For example, a bias field for a bubble device is supplied, typically, by a Watson magnet. Such a magnet comprises four elements abutting one another to form a rectangular arrangement. One pair of parallel elements comprises permanent magnets typically high coercive force INDOX. The remaining pair of elements comprises keepers of magnetically soft material such as ferrite. The magnetization of each INDOX element is saturated parallel to the faces of the element, the overall arrangement being adapted to supply a uniform field parallel to the INDOX elements within the cavity formed by the rectangular array. The plan of bubble movement is placed in that cavity and the canting of the magnet requires increased volume within the overall package and a tailored angle depending on the individual chip characteristics. The procedure is difficult to adapt to mass production.
For bubble memories in which a number of chips are organized in a single plane for encapsulation, the (Watson magnet) package becomes increasingly bulky and the difficulty in providing a uniform magnetic field increases. An alternative to the Watson magnet would be particularly attractive for producing less bulky packages if a uniform bias field still could be supplied.